1. Field of the Invention
The present invention relates to a spectrally sensitized silver halide photographic emulsion and, more particularly, to a silver halide photographic emulsion which is supersensitized using the combination of two kinds of sensitizing dyes.
2. Description of the Prior Art
It is well known that the photosensitive wave length range can be extended to a longer wave length side by adding a certain cyanine dye to a silver halide photographic emulsion, i.e., a spectrally sensitized silver halide photographic photosensitive member.
The spectral sensitivity is influenced by the chemical structure of the sensitizing dye and the properties of the emulsion such as the halogen composition, crystal property, crystal system, silver ion concentration, and hydrogen ion concentration of the silver halide, etc. Moreover, the spectral sensitivity is influenced by photographic additives such as stabilizers, antifogging agents, auxiliary coating agents, precipitating agents, color couplers and the like which are present in the emulsion.
In general, a single sensitizing dye is used in sensitizing the predetermined spectral wave length range of a photosensitive member. When sensitizing dyes are used in combination with each other, a spectral sensitivity often is obtained which is lower than that obtained using the sensitizing dyes alone. In some specific cases, however, when a specific sensitizing dye is used in combination with a specific sensitizing dye, the sensitization is increased not additively, but "superadditively". This phenomenon is known as "supersensitization". A considerable selectivity is required in combining sensitizing dyes for obtaining supersensitization. An apparently slight difference in the chemical structure markedly influences the superadditivity and thus the combination of sensitizing dyes which exhibit superadditivity is difficult to predict from the chemical structures of the sensitizing dyes.
Moreover, the sensitization of a specific emulsion can be changed by varying the state in the emulsion. For instance, the sensitization can be increased by increasing the silver ion concentration or decreasing the hydrogen ion concentration or effecting them simultaneously. Thus, the sensitization can be increased by immersing a film, on which a spectral sensitized emulsion is coated, in water or an aqueous ammonia solution. A process for further sensitizing the sensitized emulsion by increasing the silver ion concentration or decreasing the hydrogen ion concentration or effecting them simultaneously, is called hyper-sensitization. An emulsion subjected to hyper-sensitization generally has low storage stability.
Sensitizing dyes for use in supersensitizing silver halide photographic emulsions must have stable photographic properties during the storage of the photographic element without causing any mutual interaction with the photographic additives other than the sensitizing dyes.
The sensitizing dyes further must not leave any residual coloring due to residual sensitizing dyes remaining in the photographic element after processing. Particularly, no residual coloring must remain after a processing for a short period of time (generally, from several seconds to several tens of seconds).
Sensitizing dyes generally leave residual coloring in other photographic layers such as a colloidal image supporting layer and/or an auxiliary layer or a support supporting the developed and fixed photographic element. Residual coloring causes difficulties particularly in the case of a photographic element for photographic printing such as a lithographic type of photographic negative which is corrected using a color mask image, i.e., which requires a pale grey. The residual coloring effect is not desirable particularly in a photographic paper. The reason is that a light white color cannot be obtained in black and white photography, and true color reproduction is impossible.
If a yellow or magenta color remains, other defects occur. That is, the residual dye is bleached by radiated ray, thereby providing an image in which a part is white and another part is colored. Thus, it is important that the photographic paper, the lithographic type of film, and other photographic elements which are produced by using a photographic emulsion sensitized with a sensitizing dye, be free of residual coloring.
Furthermore, sensitizing dyes must provide a spectrally sensitive range sufficiently safe to a safe light which is used in processing a photosensitive element. In the case of a color photosensitive member such as color positive or color paper, a safe light having a maximum transmission in the range of about 570 nm to 620 nm is used.
In the case of an X-ray photosensitive element, such as a direct X-ray element and an indirect X-ray element, a safe light using a No. 7 filter produced by Fuji Photo Film Co., is employed. The spectral transmission curve of this filter is shown in FIG. 1.
Thus, a longer wave length side of a spectral sensitizing range of the green photosensitive layer should be cut as sharply as possible so as to make the photosensitive element stable to a safe light. If not, the safe light causes fog, thereby contaminating the finished photograph.